The present invention relates to a thin film forming method and thin film forming apparatus for forming a PZT thin film serving as a ferroelectric film containing lead (Pb), zirconium (Zr), titanium (Ti), and oxygen (O).
Ferroelectric memory elements have received a great deal of attention as a next-generation nonvolatile memory, and extensive studies have been made on them. This ferroelectric memory element is formed as a memory cell using a ferroelectric capacitor formed by sandwiching a ferroelectric film between two electrodes. A ferroelectric has “self-polarization” characteristics, i.e., characteristics in which polarization remains even at a zero voltage once a voltage is applied to the ferroelectric. A ferroelectric memory element is a nonvolatile memory using this.
A Pb(Zr,Ti)O3(PZT) film is widely used as the ferroelectric film of such a ferroelectric memory element.
The arrangement of a ferroelectric memory element using a PZT thin film will be described below. In this memory element, as shown in FIG. 9, basically, a stacked capacitor using a ferroelectric film is connected to a MOS transistor formed on a silicon substrate 901. The MOS transistor is made up of a gate electrode 903 formed on the semiconductor substrate 901 through a gate insulating film 902 and a source and drain 904 formed from diffusion layers formed on two sides of the gate electrode 903. The capacitor is made up of a Pt/TiN lower electrode 921, a PZT ferroelectric film 922 formed on the lower electrode 921, and an Ir/IrO2 upper electrode 923 formed on the ferroelectric film 922.
An interlevel insulating film 905 is formed to cover the gate electrode 903. An interconnection layer 906 serving as a bit line, which does not appear in the section of FIG. 9, is formed on the interlevel insulating film 905. An interlevel insulating film 907 is formed on the interconnection layer 906. The interconnection layer 906 is connected to one of the source and drain 904. An interconnection layer 908 is formed on the interlevel insulating film 907. The capacitor is formed on an interlevel insulating film 909 formed to cover the interconnection layer 908. The lower electrode 921 constructing the capacitor is connected to the other of the source and drain 904 through a tungsten (W) plug 910 formed in a through hole formed to extend through the interlevel insulating films 905, 907, and 909.
An insulating film 911 is formed to cover the capacitor. An interconnection layer 912 connected to the upper electrode 923 is formed on the insulating film 911.
As described above, when a ferroelectric film of PZT or the like is used, the ferroelectric film is located on an upper layer in the integrated circuit due to the following reason.
PZT is an oxide and readily reduced in an reducing atmosphere, degrading the ferroelectric characteristics. The ferroelectric characteristics readily degrade in a highly reactive environment of dry etching or the like.
A portion formed in the initial stage in the general manufacturing process of semiconductor devices tends to be exposed in a reducing atmosphere of hydrogen annealing or the like in the subsequent process. Many dry processes using plasmas, such as interconnection patterning, must be performed.
In the structure of a conventional DRAM or the like in which a capacitor is arranged immediately above a transistor, a capacitor is formed in the initial stage of the manufacture of the semiconductor device, and an interconnection structure is formed on the capacitor. A PZT film constructing a capacitor must undergo many processes in an atmosphere where the above-mentioned characteristics readily degrade.
In a ferroelectric memory element having a capacitor formed from a PZT thin film, the capacitor is located above, e.g., a multilevel interconnection structure of a semiconductor device. The capacitor is formed in the final stage in the manufacture of the semiconductor device.
As described above, since the capacitor is formed above the interconnection structure, the PZT thin film which forms the capacitor is formed at a temperature of 450° C. or less due to the following reason. The interconnection structure located below the capacitor is generally made of a metal material such as aluminum, and the metal material cannot stand at high temperatures exceeding 450° C.
There is proposed a technique of forming a PZT film at a temperature of 420° C. or less in consideration of the heat-resistant temperature of the underlying aluminum interconnection. This PZT thin film forming method will be described below. Organic metal source gases of Pb and Ti and an oxidizing gas such as N2O, O2, or NO2 are supplied at a pressure as low as 5 mTorr to a substrate heated to 445° C. A PbTiO3 crystal nucleus is formed by chemical vapor deposition (CVD) using organic metal (MO) materials. While keeping the pressure as low as 5 mTorr, organic metal source gases of Pb, Zr, and Ti and an oxidizing gas such as N2O are supplied to the substrate kept at 445° C. Since the PbTiO3 crystal nucleus is already present on the substrate, a PZT crystal grows even at the temperature as low as 445° C. to form a PZT thin film of perovskite crystal on the substrate.
According to the conventional method, a PZT thin film cannot be formed with a uniform composition on the entire substrate. The nonuniform composition results in variations in ferroelectric characteristics in the resultant PZT thin film. The variations in composition of the resultant thin film may be caused by the molecular flow of the MO source gases supplied to the substrate. Conventionally, the MO source gases are supplied at a pressure as low as several mTorr (high vacuum state) as described above. In the high vacuum state, the flow of each MO source gas becomes a molecular flow state. It is very difficult to uniformly supply each MO source gas to the substrate in this molecular flow state.